Copyright Institute of Physics 2012
Page 1
Teaching Medical Physics
Electrocardiograms
Curriculum links:
CRO traces
Frequency/Amplitude
Introduction
Electrocardiograms (ECGs) record the activity of the heart through electrodes
placed on the patient’s skin. Cardiological contraction is caused by changes in
electrical potential in the hearts muscle cells; electrical activity that the body
conducts to its surface. Although it is altered by the intervening tissue, the resulting
signal at the skin accurately reflects the cardiological cycle and can be used to
identify any anatomical and physiological anomalies in a completely non-invasive
manner.
Lesson notes
Measuring heart voltage
Non-invasive monitoring of the electrical activity of the
heart is possible because the body is an electrical conductor.
An ECG machine acts as voltmeter/CRO and voltage-time
characteristics are measured by attaching electrodes to the
patient’s skin.
In a healthy person, the resulting ECG trace has a distinctive
shape with sections that are labelled P,Q,R,S and T. Doctors
use abnormities in shape of waveform to identify heart
problems.
Structure of heart:
Pump chambers: atria and ventricles.
Control: AV and SA electrical nodes.
Blood flow
Oxygenated blood (red) flows into left chambers and
deoxygenated (blue) into right chambers.
ECG: zero potential difference across the heart
Copyright Institute of Physics 2012
Page 2
Teaching Medical Physics
Electrocardiograms
Atria
The SA node is the heart’s pacemaker. Electrical activity
originating at SA node triggers contraction of atria. Atria
pump blood through one-way valves into ventricles.
Other than the pathway through to the AV node, the atria
and ventricles are electrically insulted from each other. The
AV node momentarily delays electrical activity to allow the
ventricles to fill with blood.
ECG: contraction of atria produces P-wave
Ventricles
Electrical activity spreads from the AV node across ventricles
causing them to contract and blood is pumped through one-
way valves; oxygenated blood travels to the body and de-
oxygenated blood to lungs.
ECG: contraction of ventricles produces QRS complex
ECG trace
Final section of the ECG trace (T wave) corresponds to the
relaxation of ventricles.
Review of waveform sections:
P wave: contraction of atria
PQ interval: delay to allow ventricles to fill
QRS complex: contraction of ventricles
T wave: relaxation of ventricles
Another beat: (If required) repeat slide sequence of
heartbeat
Heart rate
The period (R-R interval) is time between adjacent peaks in
the ECG trace. In a healthy person the period varies by up to
10 % between beats. Heart rate is determined by averaging
over multiple beats
Heart rate = 1/average period
Heart rate in beats per minute (bpm) is equal to the
frequency in hertz (Hz) X 60.
Copyright Institute of Physics 2012
Page 3
Teaching Medical Physics
Electrocardiograms
Worksheet mark-scheme
1.
(a)
atrium
(b)
A
(c)
pacemaker/controls heart-rate/causes contraction of atria
[do not accept contraction of heart/ventricles]
2.
(a)
Potential difference/voltage
(b)
Evidence of measuring between three peaks
0.75 seconds [accept 0.6 or 0.8 if no average found]
(c)
Evidence of heart rate = 1/period
Answer between 75 and 95 bpm [allow error carried forward from part (a)]
(d)
P wave
missing/ECG shape abnormal
(waveform indicates) problem with atria